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1. Phosphorus content was measured in adult insects and arachnids from 170 species collected in the Sonoran Desert.

2. Across insect body sizes spanning four orders of magnitude, phosphorus content was inversely related to body mass. The largest species (similar to1 g dry) had phosphorus contents that were only about 60% (0.62% P absolute) as high as phosphorus contents of the smallest species (similar to0.0001 g dry; 0.97% P). Negative phosphorus allometry was observed within each of seven insect orders and within arachnids.

3. Phosphorus contents of insect predators and herbivores were statistically indistinguishable.

4. More recently derived orders tended to have lower phosphorus contents - with the exception of the most recently derived group (Panorpida = Diptera + Lepidoptera), which had high phosphorus contents.